1. Field of the Invention
This invention relates generally to a liquid cooled turbine bucket and more specifically to such a turbine bucket cooled by a thermosyphon effect.
2. Description of the Prior Art
Ultrahigh temperature (UHT) gas turbines operate in a range from 2500.degree. F. to 3500.degree. F., with the objective of providing as much as 200 percent more power and achieving as much as 50 percent greater thermal efficiency than conventional gas turbines. Materials employed in the manufacture of such turbines and the operating conditions therefor dictate that the buckets thereof be provided with liquid cooling.
A suitable method for liquid cooling ultrahigh temperature gas turbine blades is that afforded by open circuit liquid cooled bucket constructions such as are shown in U.S. Pat. No. 3,816,022 to Day, assigned to the assignee of the present invention. In such an open circuit liquid cooled turbine bucket construction, water or any other suitable liquid coolant is sprayed onto the sides of a turbine disk on which the buckets to be cooled are mounted. Acting under the influence of centrifugal force due to the rotation of the disk, the coolant flows radially outward collecting in gutters provided in the rim portion of the disk. The coolant flows through radially extending passages provided in the gutters, these passages communicating with coolant passages provided in the bucket below the surface thereof through flow channels disposed in the bucket platform. Again acting under the centrifugal force exerted upon the rotating bucket, the coolant flows through the subsurface channels in the bucket absorbing heat transferred to the bucket from the high temperature working fluid of the gas turbine. As the coolant flows through the subsurface channels, it is partially vaporized. The liquid and vapor components of the fluid are discharged at the tip portion of the bucket, the liquid component of the coolant moving in the generally radially outward direction through the gap between the bucket and turbine casing into a collection slot disposed within the casing.
It has been found that the cooling of ultrahigh temperature turbine buckets as afforded by the open circuit bucket construction described can be improved upon in a number of respects. For example, certain portions of ultrahigh temperature turbine buckets are heated to higher temperatures than other portions and therefore may require a higher coolant flow therethrough than other cooler portions of the bucket. However, in an open circuit liquid cooled turbine bucket having radially extending coolant channels there is no way to accurately meter varying amounts of liquid coolant to particular channels according to the temperatures to be experienced by the portions of the bucket in which those channels are disposed. Therefore, relatively large quantities of coolant must be metered to all the coolant channels to assure that none will receive too little. This may cause excessive quantities of liquid coolant which never vaporize within the bucket to be discharged from the bucket. Any liquid coolant discharged from the open circuit turbine bucket and becoming entrained in the motive fluid stream may contribute to the erosion of latter bucket stages thereby decreasing the useful life and increasing the frequency of repair or replacement of those buckets.
It will also be appreciated that the coolant discharged from an open circuit liquid cooled turbine bucket as vapor also enters the motive fluid stream and is thereby unrecoverable for condensation and reuse as a coolant. Therefore, it is also desirable to recover as much vaporized coolant exiting the turbine bucket as possible.
Liquid cooling by open circuit bucket configurations may also lower the efficiency of the turbine from the theoretical value. When the coolant is sprayed onto the turbine disk supporting the open circuit liquid cooled buckets, the fluid is accelerated to the velocity of the turbine disk. As the disk and the buckets rotate under the influence of the working fluid, they must be provided the requisite power to rotate the mass of coolant within each of the buckets. When the coolant is discharged from the bucket tip, it is moving at a velocity equal to the tangential velocity of the bucket tip. The velocity at which the coolant is discharged from the bucket is representative of what are commonly referred to as pumping losses. These pumping losses represent lost work which the turbine wheel and buckets have done on the coolant and which, if not recovered in a manner such as that disclosed in the aforementioned patent to Day are unavailable for any useful function. Therefore, to the extent they are not recovered, these pumping losses represent a diminished efficiency and decreased power output capability of the turbine.
Although thermosyphon cooling of gas turbine buckets is taught in the prior art (Mixed Convection Thermosyphon and Gas Turbine Blade Cooling -- David Japikse and Jacob Holchendler, ASME Paper 73 -- WA/HT-26, 1973) coolant vaporized within such buckets is discharged from the trailing edge of the bucket into the main motive fluid stream and is therefore unavailable for collection and reuse. Furthermore, as set forth hereinafter, constructions employed in such prior art thermosyphon cooling arrangements differ from the construction and operation of the present invention and do not provide as effective cooling as the arrangement disclosed and claimed herein.
Accordingly, it is an object of the present invention to provide a thermosyphon liquid cooled turbine bucket which minimizes the consumption of liquid coolant.
It is another object of the present invention to provide a thermosyphon liquid cooled turbine bucket wherein losses associated with the pumping of liquid coolant through the bucket are minimized.